Gas plating of aluminum using aluminum trilsobutyl



Jan. 6, 1959 w. M. MacNl-:VIN

GAS PLATING OF ALUMINUM USING ALUMINUM TRIISOBUTYL Filed Feb. e, 195e INVENTQR WILL/AM M MaclVE VIN Es ES l ATTORNEYS United States GAS PLATING OF ALUMINUM USING ALUMINUM TRIISOBUTYL William M. MacNevin, Columbus, Ohio, assignor to The Commonwealth Engineering Company of Ohio, Dayton, Ohio, a corporation of Ohio Application February 8, 1956, Serial No. 564,180

Claims. (Cl. 117-107) This invention relates to gas plating of aluminum and to'composite bodies produced thereby.

The problem of producing a coating of aluminum by volatilization of the metal and condensing the same onto a surface to be plated has failed of solution because the high temperatures involved in such a process made success irnpossible. A temperature of l800 C. (3272 F.) is required to form aluminum `metal vapor. The dilculties involved in handling such high temperature metal vapor, which has an avid tendency to react with oxygen, hydrogen or other elements presented an insoluble problem.

In accordance with the discovery of the present invention, gas plating of this light metal is carried out at relatively low temperatures and pressures by the use of a heat-decomposable gaseous aluminum alkyl, e. g., triisobutyl aluminum. The material to be plated with aluminum metal is placed in a chamber and the air is replaced with an inert gas such as nitrogen.

While thus protecting the article to be plated or aluminized, triisobutyl aluminum is introduced into the chamber, preferably utilizing an inert carrier gas such as nitrogen. Deposition of the aluminum metal is accomplished by heating the base material to a temperature suicient to decompose the aluminum triisobutyl when brought in contact with the heated article, thus releasing aluminum metal which is plated onto the surface of the article.

The present invention makes it possible to deposit aluminum on the surfaces of various materials, and such as will withstand the temperature used to bring about decomposition of the gaseous aluminum alkyl. Plating of the light metal may be made on various metal surfaces, e. g., steel, iron, copper, magnesium, or metal a1- loys as well as non-metal materials, e. g., glass, molded plastics, Wood, paper and the like.

Due to the nascent physical state of the aluminum metal, the same being deposited as it is liberated from the gaseous compound, a metal lm of one molecule thickness is provided initially which may be further increased as desired. Thus a tenacious metal layer of aluminum is formed in the surface pores of the base material on which the metal is deposited.

The process of the invention permits rapid deposition of light metals, e. g., aluminum at relatively low temperatures and pressures as compared to prior processes utilizing volatilized metals.

In gas plating metal surfaces, for example the metal, after being thoroughly cleaned of foreign matter, is subjected to gas plating utilizing a suitable organo-metallic compound of aluminum and which compound is heatdecomposable at temperatures substantially below the vaporization point of the light metal. The decomposition or disassociation of the gaseous metal compound is made to take place in an atmosphere which is inert to the nascent metal, being especially free of oxygen, hydrogen and water. An atmosphere in the plating chamber of dry nitrogen gas has been found suitable. This inert gasv Patentedv Jan.- 6, 1959 also may be used as a carrier and protecting 4blanket for the aluminum alkyl compound. Other gases which are inert and free of oxygen and moisture may be used in place of nitrogen, such as helium, argon, etc.

In carrying out the aluminum metal plating in accordance with the'preferred practice of this invention, the material to be plated is placed in a closed chamber or container having an inlet and outlet opening. Thereafter the air and water vapor are displaced by forcing dry nitrogen through the chamber under sufficient pressure to displace the air. Aluminum triisobutyl, which is containedy in a bottle or suitable container tlled with nitrogen under pressure is then introduced into the plating chamber.

To bring about decomposition of the aluminum alkyl after the same is introduced into the plating chamber, suitable heating means is provided to heat the material to be plated to a temperature highv enough to cause the gaseous aluminum triisobutyl compound to decompose and aluminum metal deposited on the surface of the material. Heating of the material may be accomplished in any suitable manner. Where the plating chamber is made of glass or plastic material which will transmit infra-red'rays, heating may be eifected by the use of infra-red lamps. Electrical resistance heating means also may be employed if desired. To this end Nichrome resistance wires or ribbons may be wound around the plating chamber enclosed in asbestos felt, or the like, and connected to a source of electricity to heat the chamber and material or article therein to bring about decomposition of the aluminum alkyl gaseous compound.

Aluminum triisobutylpreferably is conducted into the plating chamber utilizing nitrogen as a carrier gas and the ow rate is controlled whereby the air is displaced and prevented from flowing back into the plating chamber during operation of the process. For this purpose, a blower or pump may be connected to the system and operated to control the ow of gases therethrough. Gas plating of base material-with aluminum may thus be accomplished in a matter of seconds. Further, any aluminum alkyl compound which is not decomposed initially F.ice

may be recovered and recirculated back through the plating system, the same being enriched with fresh aluminum alkyl from time to time as required.

The surface of the material to` be plated with aluminum is initially cleaned of foreign matter and dried to remove moisture. Manual or chemical cleaning means or a suitable combination thereof may be employed for this purpose. Sandblasting, Wire brushing or the like are suitable methods to remove solid foreign matter from metal surfaces. Where the surface is to be de-greased and cleaned, the same may be washed with petroleum solvent, such as Xylol, toluol or the like, and all traces of moisture are removed by heating the material at 250 to 300 C. for a time sufficient to drive olf any residual moisture.

After the article has been cleaned thoroughly and freed of all moisture and rendered chemically clean, the

same is introduced into the nitrogen lled plating cham' ber and heated as described. While the article is heated in the plating chamber under an atmosphere of nitrogen,

aluminum triisobutyl is introduced and the gaseous aluminum alkyl brought in contact with the surface of the heated article, the temperature of the latter being high enough to cause the gaseous aluminum alkyl compound coming in contact therewith to decompose or disassociate and deposit aluminum metal onto the surface of the article. After a desired thickness of aluminum deposit is obtained, the process is stopped and excess aluminum alkyl returned to storage. I TriisobutylA aluminum is supplied in steel pressure bottles, with brass valves. The pressure bottle which is fittedwith a rubberdam is packed to a maximum of 60% of the free volume with triisobutylaluminum liquid, and the remaining space is pressurized to a maximum of Qpounds/sq. in. gauge with nitrogen. To remove the contents,jthe bottle is-providedI with an adapter valve and ltube outlet connection to allow the internal pressure to push out the liquid whenthe' bottle is inverted. Fittings should be opened lunder'a blanket-ofnitrogen and'receivingcontainersshouldbe dry and previously flushed with dry-nitrogen; Storage temperatures for aluminum triisobutyl are kept below 60 C. andpreferably 0 to 10 C. e A' suitableprocedure for handling and transferring a solution (47%) of aluminum triisobutylin normal heptane to a hypodermic syringe from storage pressure bottles was accomplished' as follows.

Y A gas ltight fume box, fitted with a trap chamber was constructed from clear plastic, e. g., Lucite. The box was4 provided with suitable, inlet and outlet openings for receiving tubular conduit connections, the inlet being connected to a source of nitrogen gas and nitrogen pumped into the fume box to completely displace air and water vapor. To purify the nitrogen gas, it is preferably pumped through a tower containing soda lime and calcium chlorine to remove CO2 and moisture prior to its introduction into the fume box. The auxiliary trap chamber of the fumev box allows introduction and removal of objects from the main body of the fume box Without the introduction lof air.

For removing aluminum triisobutyl from the pressure bottle, a hypodermic syringe equipped with a #1, 19 gauge needle was used. After cleaning and thoroughly drying the syringe,'. the same Was placed in the trap chamber of the fume box and the air in the syringe displaced With nitrogen by reciprocating the plunger of the syringe several times. The pressure bottle containing aluminum triisobutyl was then placed in the main cham ber of the furne box, and the syringe plunger withdrawn to ll the .syringe with nitrogen equal to the volume of liquid'aluminum alkyl to be removed from the pressure bottle. After inserting the needle of the syringe through the. rubber dam of the pressure bottle, nitrogen in the syringe was then forced into the bottle, the bottle and syringe being inverted and the plunger of the syringe slowly withdrawn to its originalposition to thus ll thev syringe with the aluminum alkyl solution. The syringe needle was then withdrawn from the dam and the pressure bottle returned to storage, the same being preferably kept at C.

` Transfer of aluminum triisobutyl solution to the decomposition or plating chamber in controlled amounts for gas plating with aluminum was accomplished. by expelling the desired amount of the aluminum alkyl from this syringe by working the plunger.

`Apparatus suitable for carrying out the gas plating with aluminum alkyl is illustrated in the drawings, whereln- Figure l illustrates an embodiment for gas plating using a` horizontally arranged plating chamber; and

Figure 2 illustrates a similar apparatus wherein a vertical gas plating chamber is utilized.

Referring more particularly to the apparatus or system tainer 10 is forced through the apparatus during plating to maintain the same free of air or oxygen. The carrier gas is oil-pumped from the storage container 10 by a pump 11 whereby the inert gas ows through conduit 12 and is discharged into the bubbler tube 13. This bubbler tube is partially lled. with normal heptane, as indicated at 1 4, and into which the discharge end of the conduit 12 is placed. Carrier gas is led from the bubbler tube 13 by a conduit 15 and thenceto azsecond bubbler tube 16 which comprises aside armV 17 tted with a rubber dam 18 for receiving a needle'19 of a plunger-actuated syringe 20. Y

The bubbler tube 16 is heated electrically by heating 4. element or pad 21, and such as formed of asbestos and Nichrome resistance wire, and which surrounds the bubbler tube. The bubbler tube 16 is similarly partially filled with normal heptane 22 as in the bubbler tube 13.

Exit gas flowing from the bubbler tube 16 is discharged from the tube through a conduit 2 4 and into a plating tube or chamber 25. The plating chamber in this instance (consists o f a hermetically sealed quartz glass tube (5% x 30) having an inlet-26 and an outlet 27. Conduit 24 is connected to the inlet 26 and the outlet 27 is connected through conduit 28 to a condenser bubbler tube 29 filled With heptane 30. The end closure plug 31 of the plating chamber is normally in place, as shown, but is adapted to be removed for inserting the material to be plated. A copper rod 32 constituting the material to be aluminum plated is suitably supported in the plating tube, as shown. To heat the plating chamber and copper rod to be plated, the, glass plating tube isl made into a resistance heater by connecting thev tube in an electrical circuit as across terminals 33 land 34.r The resistanceis made such -that a steady temperature of approximately 295 C. is reached in the plating tube. c

For condensing aluminum alkyl, forre-use, the bubbler.

tube 29 isA provided with a jacket 38 into which a suitable shown in Figure 1, nitrogen carrier gas stored in a conrefrigerant. is circulated therethrough through the inlet andfoutlet conduits 35y and 36 respectively. Waste gases are discharged from the bubbler tube 29 by a conduit 37: The condenser" bubbler tube is preferably maintained'at a temperature of about 0 C. so that any unde-y composed aluminumv alkyl is condensed and recovered fin the heptane Nitrogen carrier gas is forced: through the apparatus at a 'rate-sufficient topreventback flow of air' (oxygen) when the plating tube is opened, as when inserting material to be plated..

In the apparatus illustrated in Figure 2, a similar arrangement isused as in Figure l, except the plating tube or chamber isdisposed vertically and the .inlet for nitrogen carrier gas and aluminum alkyl is disposed in the bottom of the plating chamber, as4 shown.

In the apparatus shown, nitrogen gas stored in container 40 under pressure is pumped, as by means of an oil-pump 41through conduit 42into the bubbler tube 43 vandfinto the heptaneU 44 which partially lls the tube. Exit carrier gas from thev bubbler tube 43'is conducted through con-V duitA 4 5 to al second bubbler tube 46 which is of similar constructionas bubbler tube 16, the same being partially lill/ed` with normal heptane, designated 47, and heated byV anzelectricalrheating element 48 which surroundsv the tube. After flushing out'air and 'water vapor from the apparatus, aluminum alkyl is introduced into the bubbler tube 4,6,by means of la syringe 50, as heretofore described. The aluminum alkyl mixes with the Carrier gas and is carried alongy through conduit 5 1-toy a vertically arranged plating tube. 52V, The plating tube in this instance is madeof ordiary glass (3` x 12") and is heated by an asbestos- Nichrorne,y ribbon electrical. resistance pad 531 arranged therearound.. Electrical connection to a power source is controlledto heat the plating tube, and material to be plated to a temperature `such as to cause decomposition of thealurninum triisobutyl-and deposition of aluminum. Material to beV plated, e. g., a brass rod 55, is suitably supported inthe plating tube, as -by a hook 56. The exitend at the top of the plating tube.52 is connected with a conduitf 57 'which leads intoV acondenser bubbler tube 5 8. This bubbler tube is partially filled with normaly heptaneb as atf 59; similarly as the condenser tube of Figure 1- Reffgsraut ,iS suitably circulated, through., the jacket 6 0 sopas to; maintain the condenser. tube at a teni-l ueraiture of around .9 C- Exhauuf-,suses are @dusted from the` bubbler'ltube by vWay of conduit 6 1.

Thev following examples areillustrative but. not limita; tyu. ufvpaussss. ufl saspletiug aluuliuum .Oufu base ih .acold'luuc with thisiuteuuuu- EXAMPLE 1 Using the apparatus set up as illustrated in Figure 1, nitrogen is admitted to sweep out the air and the heating elements for the bubbler tubes and plating tubes are turned on to heat the same to around 295. The syringe is charged with 5 cc. of aluminum triisobutyl heptane solution and, after allowing about 20 minutes for the apparatus to' become free of air and the plating tube to reach 295 C., the plating is initiated by introducing aluminum triisobutyl intovthe bubbler tubeand adjusting the flow of carrier gas until heavy fumes are formed in the plating tube. Deposition of aluminum begins immediately as soon as the fumes reach the hot metal in the plating tube. After the desired thickness of aluminum plate has been formed, the addition ofaluminum alkyl is stopped and the remaining portion left in the syringe is returned to storage.

EXAMPLE II In this instance the apparatus illustrated in Figure 2 I of the drawings was used. A rod of copper was placed EXAMPLE In In this example gas plating of aluminum was carried out as described in Example I, using small steel strips as the material to be plated. A plating deposit of aluminum is formed on the steel to provide a composite steelaluminum structure.

The minimum flow rate of gas is governed by whether or not the fumes of triisobutyl reach and surround the hot metal or article to be plated. lf the flow rate is too slow, the fumes either condense on the walls of the connecting tubes of the apparatus or else a heavy layer'of white fumes accumulates in the lower half of the tube and the base metal is covered only partially. Too rapid gas ilow carries undecomposed aluminum triisobutyl out of the plating chamber, which of course, is undesirable. Also too rapid ow rate lowers the temperature inside the plating chamber and break down of the aluminum alkyl tends to occur.

The optimum rate of ow has been found to be one that will carry the aluminum alkyl into the decomposition tube or plating chamber without condensation, and which will disperse it over the whole cross section of the plating chamber so as to ll the'chamber uniformly. The ow rate in each instance was maintained high enough to keep the tube lled with aluminum alkyl fumes whereby a substantially uniform even deposit of aluminum is formed on the article being plated.

The proper carrier flow rate of nitrogen in the vertical decomposition tube is slower than that used for the horizontal tube plating chamber. In the use of the vertical tube or plating chamber the white aluminum alkyl fumes gradually build up from the bottom and tend to till the entire space. The rate of flow that need be maintained is thus adjusted whereby it is just enough to keep the Vertical tube or plating chamber lled with aluminum alkyl fumes without building up of fumes in the lower portion of the tube.

The minimum temperature that will vaporize aluminumtemperature. i It has been found by test runs, however, that'temperatures between 275 C. and 325 C. give good' decomposition of the aluminum alkyl and quality deposits of aluminum. At 350 C. the aluminum deposit on copper metal is of dark bluish color, and a clear reecting mirror is formed on the inside of a glass tube used as a plating chamber. When the plating temperature wasraised to 400 C., the mirror-like deposit formed was observed to be dark in color and not as good` as where a lower temperature was used.

Triisobutyl aluminum, as supplied by the Hercules Powder Company is in liquid form. The composition as shipped consists of 60% triisobutyl aluminum:

Al [CH2-CH-CH3 2l 3, 40% i diisobutyl aluminum hydride: Al (H) [CH2-CH-(CH3 2] 2.

Physical characteristics Diisobutylalumlnum hydride Chemical characteristics It flames upon contact with moisture or moist air; even extremely rapidly oxidized in dry air. Reacts vigorously with compounds containing active hydrogen.

. Stable below 60 C.; a sample stored at 60 C. for 72 hours developed two pounds per square inch gage pressure on bringing to temperature. Pressure remained steady for the rest of the time.

Triisobutylaluminum can be distilled with care to keep boiling temperature below C. For instance, a boiling point of y52 C. has been recorded at 0.5 mm. Hg. lf small amounts of isobutylene split oil, they will go over into the cold trap and the distillate will remain pure. VThe residual monohydride will remain in the high boiling residue. v

The plated materials, if desired, may be subjected to an annealing heat treatment after the plating operationA to stabilize the metal deposit. Stich a heat treatment may consist of heating the plated article up to 400 C. for one hour.

` The plated materials, if desired, may be subjected to an annealing heat treatment after the plating operation to stabilize the metal deposit. Such a heat treatment may consist of heating the plated article up to 400 C. for one hour.

The invention provides a process for plating aluminuml metal directly on the surface of articles whereby there is produced a composite' product comprising an outerlayer `of aluminum or light metal intricately and tenaciously bonded thereto.

The thickness'of the light metal coating may be controlled by varying the time the metal bearing gas is in contact with the heated material to be plated. A coating of one molecule thickness up to one or more thousandths of an inch may be effected utilizing the process of the invention to thus provide a protective coating of aluminum on various metal surfaces or the like, the invention being especially useful to overcome corrosion problems.

The process further makes possible the continuous straight line production of composite products; and long length strips or sheets of material, e. g., such as used in the making of screens and the like which may be gas plated with aluminum. The invention is particularly useful in aluminizing metals where it is desired that aluminum be deposited in the pores and interstices of the base metal surface to form a substantially integral outer shell of the corrosion resistant light metal.

l `Preheating and drying of the article .prior togas platingpreferably is carried out in an inert atmosphere such as nitrogen gas, helium and the like to prevent oxidation. Thereafter the heated and completely dry moisture free article is subjected to gas plating with gaseous aluminum triisobutyl as described.

The aluminized metal articles, as plated, are free of oxides, but upon its removal from the plating chamber and subjection to the surrounding atmosphere, an outer coating of aluminum oxide is immediately formed. This, however, does not detract from the process inasmuch as this oxide film on aluminum metal functions to protect the same as is well known.

lt will be understood that while there has been described and set forth certain specific embodiments of this invention, it is not intended that the invention be thus limited thereto and it is manifest that various substitutions and changes may be made by those skilled in the art and to'which the invention is readily susceptible without departing from the spirit and scope of this disclosure and as more particularly set forth in the appended claims.

What is claimed is:

, l. A method of aluminum plating material which comprises' establishing a source of dry inert gas free of oxygen and moisture, enclosing said material to be plated in a plating chamber filled with said inert gas, introducing aluminum triisobutyl into said plating chamber and in contact with said material, and heating said material to a temperature to cause decomposition of said aluminum triisobutyl and deposition of aluminum onto the surface of said material.

2. A method ofaluminum plating material which cornprises establishing a source of dry nitrogen gas free of oxygen and moisture, enclosing said material to be plated in an atmosphere of said dry nitrogen, introducing aluminum triisobutyl into said plating chamber and in contact with said material, and heating said material to a temperature to cause decomposition of said aluminum triisobutyl and deposition `of aluminum onto the surface of said material.

3. A method of aluminum plating material which comprises establishing a source of dry nitrogen gas free of oxygen and moisture, enclosing said material to be plated in an atmosphere of said dry nitrogen, introducing aluminum triisobutyl into said plating chamber and in contact with said material, and heating said material to a temperature to cause decomposition of said aluminum triisobutyl and deposition of aluminum onto the surface of said material, said nitrogen gas being admixed with gaseous aluminum triisobutyl and conducted into said plating chamber and into contact with the heated material to be plated.

4. In a method of depositing aluminum on an article, the step of passing over said article vapors of aluminum triisobutyl together with an inert carrier gas free of oxygen and moisture heated to a temperature suicient to cause said vapors to decompose and aluminum metal to be deposited onto said article.

5. In a method of depositing aluminum on an article, the step of passing over Asaid article vapors of aluminum triisobutyl together with ran inert carrier gas free of oxygen and moisture heated to a temperature of about 295 C. and 450 C. suflicient to Vcause said vapors to decompose and aluminum metalv to be deposited onto said article.

6. A method of aluminum plating a metallic object which comprises the steps of heating the object to a temperature of about 400 C., vaporizing aluminum triisobutyl, and contacting the heated object With the vaporized aluminum alkyl compound.

7. A method of aluminum plating a metallic object which comprises the steps of heating the object to a temperature of about 400 C., vaporizing aluminum triisobutyl with nitrogen free of oxygen and moisture at a temperature of about 300 C., and flowing the gaseous mixture of nitrogen and aluminum triisobutyl into contact with said heated object.

8. A method of plating a metallic object with aluminum which comprises the steps of heating the object to be plated in an atmosphere of dry nitrogen free of oxygen v and moisture, said article being heated to a temperature suilcient to cause gaseous aluminum triisobutyl brought in contact therewith to decompose, and then contacting said heated article with said gaseous aluminum compound while enclosed in an inert atmosphere and at substantially atmospheric pressure.

9. A method of plating a metallic object with aluminum which comprises the steps of heating the object to be plated in an atmosphere of dry nitrogen free of oxygen and moisture, said article being heated to a temperature sufcient to cause gaseous aluminum triisobutyl brought in contact therewith to decompose, and then contacting said heated article while retained in a dry atmosphere of inert gas with a gaseous mixture comprising nitrogen and aluminum triisobutyl and at substantially atmospheric pressure.

10. As an article of manufacture, a gas plated product made in accordance with claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 1,497,417 Weber ,June l0, 1924 2,643,959 Fischer June 30, 1953 2,694,651 Pawlyk Nov. 16, 1954 FOREIGN PATENTS 122,717 Australia Nov. 21, 1946 722,797 Great Britain Feb. 2, 1955 OTHER REFERENCES The Journal of Chemical Physics, vol. 14, No. l (January 1946), pp. 1-7 relied on. (Copy in Div. 70.) 

1. A METHOD OF ALUMINUM PLATING MATERIAL WHICH COMPRISES ESTABLISHING A SOURCE OF DRY INERT GAS FREE OF OXYGEN AND MOISTURE, ENCLOSING SAID MATERIAL TO BE PLATED IN A PLATING CHAMBER FILLED WITH SAID INERT GAS, INTRODUCING ALUMINUM TRIISOBUTYL INTO SAID PLATING CHAMBER AND IN CONTACT WITH SAID MATERIAL, AND HEATING SAID MATERIAL TO A TEMPERATURE TO CAUSE DECOMPOSITION OF SAID ALUMINUM TRIISOBUTYL AND DEPOSITION OF ALUMINUM ONTO THE SURFACE OF SAID MATERIAL. 